This invention relates to fluid flow control systems and more particularly to gaseous fuels systems for internal combustion engines.
With ever more stringent emissions regulations, the use of alternative gaseous fuels for operating internal combustion engines has become increasingly attractive. The more commonly used gaseous fuels are compressed natural gas (CNG) and liquefied petroleum gas (LPG). These gases can burn cleaner than gasoline and the cost per unit of energy is lower.
Currently, the gaseous fuels industry still relies heavily on carburetor based mixture formation devices, of the fixed or variable venturi type. There are several major shortcomings associated with the aforementioned technologies, the most serious of which are the airflow restriction created by the venturi and the closed loop control problems. For a given air flow through a venturi, the venturi draws a predetermined amount of fuel, the fuel flow rate being a function of the depression at the venturi throat. In order to alter the fuel flow rate, additional devices are required that need to work in co-operation with the venturi. Due to the complexity of the flow phenomena through the venturi, carburetors are difficult to integrate into control systems.
Since there is no need to atomize a gaseous fuel, a gas metering device of adequate control range, speed and resolution would in principle suffice. Such metering devices have been developed, utilizing either a rotary butterfly valve, or a linear conical valve, actuated by a proportional solenoid.
The drawbacks of the butterfly valve solution are the impossibility to seal fuel flow completely, a usable angular stroke lesser than 90 degrees and the difficulty in profiling the valve to match fuel flow requirements.
A linear conical valve device can be designed to seal when closed and the cone profile can be shaped to match fuel flow requirements. There are, however, at least two main problems with the linear type actuator required to drive such a valve: in order to increase control resolution, it is desirable to extend the cone profile as much as possible. This entails a long actuator stroke, which leads to large, heavy and expensive actuators. The other problem is the difficult and expensive means to measure the linear position of the conical valve.